Nuclear reactors are cooled by circulation of a fluid, for example water, which must be put into circulation between the reactor and steam generators. This circulation of the cooling fluid is assured by primary motor pumps which are usually of vertical-axis type and which comprise essentially a drive motor, a flywheel and the pump proper. The latter comprises a wheel forming a rotor which is fed axially by an inlet neck and which is driven in rotation inside a stator which may form an outlet diffusor which opens out inside a flow-chamber. In recent designs the flow-chamber of toroidal shape surrounds the inlet neck. The rotor located at the end of the inlet neck is mounted overhung at the end of a drive shaft which extends in prolongation of the axis of the inlet neck and passes through a cross wall which defines the flow chamber at the side opposite from the inlet neck. The rotor is composed of a plurality of blades fixed between a disc mounted at the end of the drive shaft and an outer shroud of toroidal shape, and it revolves inside a stator in the form of an outlet volute which comprises a plurality of fixed blades arranged between the cross wall of the flow-chamber and an outer toroidal shroud located in prolongation of the outer shroud of the rotor. The latter is surrounded by a cylindrical part which connects the end of the inlet neck to the stator shroud as to assure continuity of guidance of the fluid.
In general, a labyrinth seal located at the inlet to the rotor assures fluidtightness between the high pressure and low pressure portions of the pump, consisting respectively of the flow chamber and the inlet neck, the fluid tightness at the entry of the drive shaft into the flow-chamber being assured by revolving fluid tight seals of conventional type. The bottom bearing of the line of shafts, which may have three or four bearings, is normally located inside the fluid tight enclosure thus defined. Hitherto the rotor has always been mounted overhung at the end of the drive shaft, so that the distance between the bottom bearing and the center of gravity of the wheel may be more or less considerable, depending upon the type of bearing chosen.
For example, it is fairly usual in the primary pumps of pressurized-water or boiling-water reactors to employ a hydrodynamic bearing, but this bearing for satisfactory operation calls for the viscosity of the water to be kept above a certain value. On the structural plane, this obligation is expressed by the installation, between the bearing and the wheel, of the heat barrier in the form, for example, of an exchanger, which increases the overhang. In order to avoid this disadvantage, it is possible to employ a hydrostatic bearing, the operation of this type of bearing not being dependent upon the value of the viscosity of the fluid. In this case the hydrostatic bearing is located just above the rotor; the overhang which is equal to the distance between the centre of the bearing and the center of gravity of the rotor is therefore reduced but still remains considerable. On the other hand, and heat barrier which remains necessary for the protection especially of the revolving fluid tight seals is located above the hydrostatic bearing and hence is remote from the cross wall of the flow chamber.